1. Field of the Invention
The present invention relates to isolated DC-DC converters that digitally control switching circuits.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2000-116027 describes a known isolated DC-DC converter for digital control including a control circuit configured using a microcomputer or a DSP (digital signal processor) to maintain the output voltage at a secondary side at a constant value.
Since the microcomputer or the DSP can communicate signals and data with an external circuit, the performance of the converter can be improved when the microcomputer or the DSP is used for switching control.
Herein, FIG. 1 illustrates a basic structure of the isolated DC-DC converter described in Japanese Unexamined Patent Application Publication No. 2000-116027.
In FIG. 1, a switching element 211 and a resistor 257 for current detection are connected in series to a primary winding of a transformer T10. The switching element 211 is configured to receive driving signals from a DSP 903. A rectifying/smoothing circuit 208 is connected to a secondary winding of the transformer T10. The DSP 903 includes a photocoupler 702a, 702b that detects the output voltage of the rectifying/smoothing circuit 208 and transmits the voltage to a primary side of the transformer while isolating a secondary side of the transformer from the primary side and an A/D converter 255 that converts the output voltage into digital data. The DSP 903 reads the values. Moreover, the DSP 903 reads the dropped voltage of the resistor 257 for current detection. A photodetector 902 (that receives infrared signals from a remote controller) is disposed at the primary side of the transformer T10, and the DSP 903 reads output signals from the photodetector 902.
In this manner, the DSP 903 controls the switching of the switching element 211 such that the output voltage is maintained at a constant value, and protects the converter from overcurrent in accordance with the output voltage of the resistor 257. Furthermore, the DSP 903, for example, starts and stops the converter in accordance with the signals from the photodetector 902.
Since a known isolated DC-DC converter maintains the output voltage at the secondary side of a transformer at a constant value by switching a switching element at the primary side of the transformer, the converter generally has a digital controller (a microcomputer or a DSP) disposed at the primary side as shown in FIG. 1. Since the primary side and the secondary side need to be isolated from each other in the isolated converter, signals of the output voltage detected at the secondary side are transmitted to the primary side using isolation means such as a photocoupler, and are fed back to the digital controller so that the switching element at the primary side is controlled.
However, when the output voltage at the secondary side is transmitted to the primary side using a photocoupler, problems such as delay in response and deterioration with time may arise. Moreover, when the digital controller is disposed at the primary side, the digital controller cannot communicate with a load (electronic device) to which the output voltage at the secondary side is applied. That is, the digital controller using a microcomputer or a DSP as a controller can constantly send information such as the state of a power supply unit to the electronic device serving as the load, and can receive information such as the load state of the electronic device for constant current control, constant voltage control, constant power control, and the like, without a time lag. However, when the digital controller is disposed at the primary side, the above-described advantages cannot be obtained.
As a matter of course, isolation means such as a photocoupler or a pulse transformer technically allows the above-described communication. However, a number of photocouplers, pulse transformers, or the like are required in accordance with the number of ports for signals that are mutually communicated, which is impractical from the viewpoint of reducing cost and saving space.
Conversely, when the digital controller is disposed at the secondary side, a problem associated with detection of the input voltage arises. That is, since the input voltage fluctuates under normal conditions, the input voltage needs to be detected at the secondary side via the above-described isolation means. Furthermore, another isolation means is required for transmitting control signals for the switching element at the primary side. This disadvantageously requires an increase in the size of the converter.
As described above, since the values to be detected (for example, the input voltage and the output voltage) exist at both the primary and secondary sides, signals need to be transmitted using isolation means such as photocouplers or pulse transformers according to the known technology regardless of whether the digital controller is disposed at the primary side or the secondary side. This causes problems such as delay in response, deterioration with time, and an increase in the size of the converter.